1. Field of the Invention
This invention relates to carbon nanotubes, and particularly to methods for separating carbon nanotubes by enhancing the density differentials of the nanotubes.
2. Description of Background
One-dimensional nanostructures such as nanowires and carbon nanotubes (CNTs) exhibit excellent electrical properties and thus serve as attractive materials for use in electronic devices, including field effect transistors (FETs), memory elements, and sensors. The electronic properties of carbon nanotubes, however, depend highly on diameter and chirality, which controls whether a carbon nanotubes is metallic or semiconductive. For example, the band gap of semiconductive carbon nanotubes scales approximately with the inverse of its diameter. Thus, to produce CNT devices with select electrical characteristics, it is desirable to be able to tightly control the diameter distribution of the carbon nanotubes present in such devices.
Current methods employed to synthesize carbon nanotubes yield a complex mixture of both metallic and semiconductive carbon nanotubes. Unfortunately, efforts to separate carbon nanotubes by diameter and chirality have met with mixed success. For example, nanotubes of different diameters can be separated by functionalizing the nanotubes with surfactants such as deoxyribonucleic acid (DNA) strands, suspending the functionlized nanotubes in an aqueous solution, and then subjecting the functionalized nanotubes to high-speed centrifugation. Consequently, the nanotubes become separated based on the intrinsic density gradient created when the nanotubes are suspended in an aqueous solution followed by centrifugation. However, the difference in mass, and therefore difference in density, among the nanotubes is small, making this separation difficult.